Random access memories (RAMs) typically include a plurality of memory cells arranged in an array of rows and columns. Data are accessed from the memory cells by selecting a given cell or cells by activating bit lines and word lines according to a row and column address. Due to the need to minimize array sizes, memory cells are fabricated with minimal dimensions, and consequently, the data signal provided by the memory cells is relatively small. Commonly, RAMs include a bank of sense amplifiers for receiving the data signals from the memory cells, and amplifying it for use in subsequent data input/output (I/O) path. Because data sensing speeds are a large portion of overall memory device access time, sense amplifiers and I/O paths play a critical role in the overall RAM speed.
U.S. Pat. No. 4,608,670 issued to Duvvury et al. on Aug. 26, 1986, U.S. Pat. No. 4,627,033 issued to Hyslop et al. on Dec. 2, 1986, and U.S. Pat. No. 5,127,739 issued to Duvvury et al., on Jul. 7, 1992 disclose a CMOS sense amplifier having cross-coupled n-channel transistors and cross-coupled p-channel transistors connected between a bit line pair. The sources of the p-channel transistors are coupled to a positive power supply by one or more p-channel coupling devices and the sources of the n-channel transistors are coupled to a low power supply by a number of n-channel coupling devices. In a sensing operation, once a data signals is placed on the bit lines, the first, relatively small n-channel coupling device is activated and the cross-coupled n-channel pair begin pulling one of the bit lines to ground. Subsequently, the second, relatively larger n-channel coupling device is activated, and the cross-coupled n-channel pair begin pulling the low-going bit line to ground at an even faster rate. Finally, either concurrently with, or subsequent to the activation of the larger n-channel coupling device, the p-channel coupling device is activated and the bit line that is not low-going is pulled to Vcc by the p-channel cross-coupled pair. The n and p-channel coupling devices can be shared by all the sense amplifiers in a row, or unique to each sense amplifier. The '739 patent further discloses an arrangement wherein the sense amplifiers can be isolated from the bit lines by isolation transistors.
As RAM sizes continue to grow larger and system clocks continue to get faster, prior art solutions to increasing sensing speed and decreasing sensing current are no longer adequate. It would be desirable to provide a sensing arrangement having sensing speed and sensing current performance beyond that provided by the prior art.